1. Field of the Invention
The present invention relates to interface circuits for receiving high data rate signals from long lengths of cable, and in particular, interface circuits for receiving high data rate, baseband, binary encoded data signals from long lengths of cable.
2. Description of the Related Art
In a typical high speed digital wire-line communication system, the channel introduces frequency dependent loss. These losses cause inter-symbol interference (ISI) when the channel is conveying a random data pattern. An equalizer removes the ISI by implementing the inverse channel response that compensates for the signal distortion caused by the channel. An adaptive equalizer automatically compensates for the loss of the channel.
Recovering data which has been transmitted over a long length of cable at high rates requires that such data be equalized in order to compensate for the loss and phase dispersion of the cable. Further, in those applications where the cable length may vary, such equalization must be based upon a complementary transfer function which is capable of adapting accordingly since the transfer function of the cable varies with the length of the cable. This equalizing is generally done using three functions: a filter function; a dc restoration and slicing function; and an adaptation control, or servo, function.
The filter function is performed using a complementary (with respect to the complex cable loss characteristic) filter which synthesizes the inverse of the transfer function of the cable. Since the bit error rate (BER) is directly related to jitter, an important performance metric for an equalizer is jitter within the output waveform. The extent to which the equalizer is able to match the inverse of the complex cable loss characteristic determines the extent to which inter-symbol interference induced jitter is eliminated.
Conventional equalizers use gm/C types of continuous time filters or finite impulse response (FIR) filters. However, these types of filter structures tend to be complex and have difficulty maintaining the required balance among the desired operating characteristics, such as output jitter, compensation for process and temperature variations, and optimization of the signal-to-noise ratio (SNR).